/*
 * Copyright 2002-2018 the original author or authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      https://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.springframework.aop.interceptor;

import org.aopalliance.intercept.MethodInterceptor;
import org.aopalliance.intercept.MethodInvocation;
import org.springframework.aop.Advisor;
import org.springframework.aop.support.DefaultPointcutAdvisor;
import org.springframework.core.NamedThreadLocal;
import org.springframework.core.PriorityOrdered;

import java.io.Serializable;

/**
 * Interceptor that exposes the current {@link org.aopalliance.intercept.MethodInvocation}
 * as a thread-local object. We occasionally need to do this; for example, when a pointcut
 * (e.g. an AspectJ expression pointcut) needs to know the full invocation context.
 *
 * <p>Don't use this interceptor unless this is really necessary. Target objects should
 * not normally know about Spring AOP, as this creates a dependency on Spring API.
 * Target objects should be plain POJOs as far as possible.
 *
 * <p>If used, this interceptor will normally be the first in the interceptor chain.
 *
 * @author Rod Johnson
 * @author Juergen Hoeller
 */
@SuppressWarnings("serial")
public final class ExposeInvocationInterceptor implements MethodInterceptor, PriorityOrdered, Serializable {

    /**
     * Singleton instance of this class.
     */
    public static final ExposeInvocationInterceptor INSTANCE = new ExposeInvocationInterceptor();

    /**
     * Singleton advisor for this class. Use in preference to INSTANCE when using
     * Spring AOP, as it prevents the need to create a new Advisor to wrap the instance.
     */
    public static final Advisor ADVISOR = new DefaultPointcutAdvisor(INSTANCE) {
        @Override
        public String toString() {
            return ExposeInvocationInterceptor.class.getName() + ".ADVISOR";
        }
    };

    private static final ThreadLocal<MethodInvocation> invocation =
            new NamedThreadLocal<>("Current AOP method invocation");


    /**
     * Ensures that only the canonical instance can be created.
     */
    private ExposeInvocationInterceptor() {
    }

    /**
     * Return the AOP Alliance MethodInvocation object associated with the current invocation.
     *
     * @return the invocation object associated with the current invocation
     * @throws IllegalStateException if there is no AOP invocation in progress,
     *                               or if the ExposeInvocationInterceptor was not added to this interceptor chain
     */
    public static MethodInvocation currentInvocation() throws IllegalStateException {
        MethodInvocation mi = invocation.get();
        if (mi == null) {
            throw new IllegalStateException(
                    "No MethodInvocation found: Check that an AOP invocation is in progress, and that the " +
                            "ExposeInvocationInterceptor is upfront in the interceptor chain. Specifically, note that " +
                            "advices with order HIGHEST_PRECEDENCE will execute before ExposeInvocationInterceptor!");
        }
        return mi;
    }

    @Override
    public Object invoke(MethodInvocation mi) throws Throwable {
        MethodInvocation oldInvocation = invocation.get();
        invocation.set(mi);
        try {
            return mi.proceed();
        }
        finally {
            invocation.set(oldInvocation);
        }
    }

    @Override
    public int getOrder() {
        return PriorityOrdered.HIGHEST_PRECEDENCE + 1;
    }

    /**
     * Required to support serialization. Replaces with canonical instance
     * on deserialization, protecting Singleton pattern.
     * <p>Alternative to overriding the {@code equals} method.
     */
    private Object readResolve() {
        return INSTANCE;
    }

}
